1. Field of the Invention
The present invention relates to an all-optical switch apparatus using a nonlinear etalon in which a control beam turns a signal beam on and off, or switches the direction of the signal beam.
2. Prior Art
One of the simplest of conventional optical switches is shown in FIG. 1. This optical switch, employing an electronic circuit that controls a signal beam by means of an electric signal, first converts signal beam Ps into an electric signal by means of photodiode 1, and then modulates, switches, or amplifies the electric signal by electric circuit 2, and finally converts the electric signal into an optical beam by means of laser diode 3, producing the optical beam as output beam Po.
Another conventional optical switch, which switches the traveling direction of the signal beam, is shown in FIG. 2. This switch detects signal beam Ps by means of photodetector 1, such as a photodiode, converts the signal beam Ps into an electric signal, amplifies the electric signal by means of electric circuit 2, and selectively drives diode laser 5 or 6 according to the electric signal and control voltage Vc. Thus, input signal beam Ps is outputted from first waveguide 8a or second waveguide 8b.
Another switch that switches the direction of the signal beam is shown in FIG. 3. It uses optical directional coupler 7, and has input signal beam Ps selectively emitted from first waveguide 8a or second waveguide 8b according to the applied control voltage Vc.
Furthermore, other types of switches can be employed. Among these, are electrically controlled switches using waveguides made of materials (such as lithium niobate, gallium arsenide, YIG) exhibiting electro-optical effects, or materials exhibiting acousto-optical effects, magneto-optical effects, or thermooptical effects.
These optical switches turn on and off by changing the state of the optical switching material, or by changing the refractive index or the absorption coefficient thereof by controlling the voltage or current. The switching speed of these types of switches is restricted by the bandwidth of the electric circuit as well as by the response time of the materials used in the switch, which impose an undue limit on the bandwidth of the light. In addition, a wideband electronic circuit is required, which places an additional demand on the electronic circuit.
For these reasons, an all-optical switch, utilizing a nonlinear optical material and controlled by an optical beam, is required for high-speed switching. A nonlinear etalon (Fabry-Perot etalon) is a promising type of basic switching element for all-optical switches which can utilize the properties of nonlinear optical materials.
Nonlinear etalons have a pair of mirrors between which a nonlinear optical material is inserted, and have a sharp resonance transmission peak at a wavelength corresponding to the light-pass length between the mirrors. The optical resonator length of the etalon varies with the intensity of incident light because the refractive index of the nonlinear medium changes in response to an increase or a decrease in the intensity of incident light. Consequently, the nonlinear etalon has a nonlinear input-output characteristic corresponding to the operational detuning around the resonance wavelength. The operational detuning is defined as the difference between the resonance wavelength of the etalon and the wavelength of incident light. Optical switching can be achieved by using the above characteristic. Nonlinear etalons themselves are well known, and many papers describing them have been published. The major results of these papers are covered in the book by H.M. Gibbs entitled "Optical bistability: controlling light with light" (Academic Press, New York, 1985).
Conventional optical switches employing nonlinear etalons, however, have several disadvantages that prevent practical use. The following are some of the problems of conventional optical switches using nonlinear etalons:
(1) FIG. 4 shows an optical switch that combines signal beam Ps and control beam Pc by means of prism 10, inputs the beams normally to nonlinear etalon 12 via lens 11, and emits the output light of nonlinear etalon 12 through lens 13. The optical switch presents the following problems:
(1a) The operational detuning of nonlinear etalon 12 must be specified to an accuracy better than 10.sup.-1 of the full width at half maximum (FWHM) to obtain a desired input-output characteristic of nonlinear etalon 12. The operational detuning, as mentioned above, is the difference between the wavelength of incident light and the wavelength at the center of a resonant peak of etalon 12. Let us consider the case where the resonance wavelength of etalon 12 is specified according to the wavelength of a beam produced from a given optical source. As a typical case, assuming that the wavelength of the beam from the optical source is 1 .mu.m, and a full width at half maximum (FWHM) of etalon 12 is about 1 nm, then the accuracy of the thickness required of the nonlinear medium of etalon 12 must be better than 10.sup.-4. The current technique cannot achieve such a high accuracy with a high yield. This problem has not yet been solved, and recent reports such as "Low-power optical bistability in a thermally stable AlGaAs etalon" by E. Masseboeuf, et al. (Appl. Phys. Lett. 54(23), pp. 2290-2292, 5 June 1989) describes a method for adjusting operational detuning by modifying the wavelength of light by using a light source of variable wavelength such as a dye laser. However, selecting a light source according to a switch is obviously impractical.
(1b) The arrangement in FIG. 4 employs individual components such as lenses 11 and 12, prism 10, and nonlinear etalon 12, which are not assembled into one unit. As a result, the adjustment of the alignment is difficult. In addition, high reliability after adjustment cannot be ensured.
(1c) A high on/off ratio of switching cannot be obtained, because signal beam Ps and control beam Pc are not separated, which is particularly true when signal beam Ps is weak.
(2) FIG. 5 shows an arrangement in which signal beam Ps and control beam Pc are obliquely inputted to nonlinear etalon 12 through lenses 15 and 16, so that the operational detuning of nonlinear etalon 12 is adjusted by the angle of incidence, and the output beam from the etalon is emitted through lens 18 as output beam Po. Such an arrangement is described, for example, in "Thermally induced optical bistability in thin film devices" by I. Janossy, et al. (IEEE Journal of Quantum Electronics, Vol. QE-21, pp. 1447-1452, 1985), in which the operational detuning is adjusted by means of the angle of incidence. However, this method presents the following problems:
(2a) Adjustment of the incident angle is difficult while maintaining the focal point of beams Ps and Pc on etalon 12. In particular, it is difficult to maintain optical fibers (guiding signal beam Ps and control beam Pc), lenses 15 and 16, and nonlinear etalon 12 in coupling arrangement.
(2b) The focal lengths of lenses 15 and 16 must be long owing to their sizes. As a result, the spot sizes of beams Ps and Pc on etalon 12 must be relatively large, and therefore the intensity of light in the spot is low, and so the beam intensity must be increased.
(3) U.S. Pat. No. 4,630,898 entitled "Etalon optical logic gate apparatus and method" describes logic-arithmetic elements using a etalon. The logic gate disclosed employs control beams (input beams) and a signal beam (probe beam), and changes the transmittance and reflectance of the etalon by means of control beams in order to control the passing of the signal beam through the etalon to perform a logical operation. In this case, the wavelengths of the control beams and that of the signal beam are different: the wavelengths of the control beams are such that the wavelengths can alter the transmittance and reflectance of the etalon; whereas the wavelength of the signal beam is such that the wavelength does not substantially alter the transmittance and reflectance of the etalon. In addition, the wavelengths of control beams are rather apart from the wavelength of the resonant peak of the etalon. Disadvantages of the apparatus and method are as follows:
(3a) As described in (1a) above, the operational detuning of nonlinear etalon 12 must be specified with an accuracy greater than 10.sup.-1 of full width at half maximum (FWHM) to obtain a desired input-output characteristic thereof (the operational detuning of nonlinear etalon corresponds to the difference between the resonance wavelength of etalon and that of the probe beam in the U.S. Pat. No. 4,630,898). In a typical case, the thickness accuracy required of the nonlinear medium of nonlinear etalon 12 must be greater than 10.sup.-4, as described above. The current technology cannot achieve such a high accuracy with a high yield.
(3b) As described in (1b) above, the arrangement in FIG. 1 in U.S. Pat. No. 4,630,898 employs individual components, such as lenses, a polarizing cube, a nonlinear etalon, etc., which are not assembled into one unit. As a result, the adjustment of the arrangement is difficult. In addition, high reliability after adjustment cannot be ensured.